Sulfur-covalent

Strain energies of 23.5, 24.8 and 8.3 kJ mol 1 were estimated for tetrahydrofuran, pyrrolidine and tetrahydrothiophene, respectively (74Pmh(6)199). The larger sulfur covalent radius of 1.04 A lowers angular strain. 

In glucosides, hemiacetal hydroxyl activation/substitution can be achieved using a sulfonic anhydride and a nucleophile, plus a base as acid scavenger.12 The reaction is catalysed by dibutyl sulfoxide (Bu2S=0), and shows evidence of sulfur-covalent catalysis. Using benzenesulfonic anhydride [(PhS0)20], it is proposed to involve initial formation of a sulfonium sulfonate (6), the S(IV) centre of which then reacts with... 

There have been a considerable number of MO calculations performed on four-coordinate square-planar metallo-bis(dithiolene) complexes at various levels of theory (283, 327, 328, 379-384). The most important differences in the results of these various calculations rests in the energy level ordering of the valence MOs as well as the degree of metal-sulfur covalency. This point is important as the nature of the MO scheme has greatly affected how the results of both ground and excited-state spectroscopic studies on very similar... 

Fig. 23. Proposed mechanisms for Tyr-Cys cofactor biogenesis. (A) Initial Cu(II) binding leads to the appearance of resonance contributions from a reactive phenoxyl that electrophilically attacks the neighboring Cys-228 thiol. (B) Cu(I) binding produces an oxygen-reactive complex that drives hydrogen abstraction from Cys-228 to form a thiyl free radical which subsequently attacks the neighboring Tyr-272 ring with formation of a carbon-sulfur covalent bond.

Figure 5 Schematic of metal - sulfur covalency that can occur upon dithiolate folding...

Rose K, Shadle SE, Eidsness MK, Kurtz DM, Scott RA, Hedman B, Hodgson KO, Solomon El (1998) Investigation of iron-sulfur covalency in rubredoxins and a model system using sulfur K-edge X-ray absorption spectroscopy. J Am Chem Soc 120 10743-10747... 

The sums of the respective covalent and van der Waals radii for Hg(II) and relevant donor atoms are given in Table II. An additional 0.2 A is added to the bridging sulfur covalent radii compared to terminal sulfur radii, as suggested by Bowmaker et al. (23). Although several values for the van der Waals radius of Hg(II) (29, 52, 12, 147) have been reported, the most recent work by Canty and Deacon proposed a value of 1.73 A with an acceptable range from 1.70 to 2.00 A, which is used in this chapter... 

The tertiary structure of many proteins is strengthened by sulfur-sulfur covalent bonds called disulfide linkages. The amino acid cysteine contains a sulfur-hydrogen bond. Two cysteines in a protein can form a disulfide linkage, which stabilizes the protein structure much like a cross-link strengthens a polymer s stracture. Many other noncovalent intermolecular forces contribute to the stability of the tertiary structure of a protein, as shown in Figure 22.6. 

Alkanethiols and other sulfur-bearing hydrocarbons covalently attach to metal surfaces alkanethiol onto gold is the most widely studied of these systems [27-29,31,32,45]. These SAMs are ordered provided the alkane chain contains nine or more carbons [32]. Binary solutions of two alkanethiols also appear... 

The most popular device for fluoride analysis is the ion-selective electrode (see Electro analytical techniques). Analysis usiag the electrode is rapid and this is especially useful for dilute solutions and water analysis. Because the electrode responds only to free fluoride ion, care must be taken to convert complexed fluoride ions to free fluoride to obtain the total fluoride value (8). The fluoride electrode also can be used as an end poiat detector ia titration of fluoride usiag lanthanum nitrate [10099-59-9]. Often volumetric analysis by titration with thorium nitrate [13823-29-5] or lanthanum nitrate is the method of choice. The fluoride is preferably steam distilled from perchloric or sulfuric acid to prevent iaterference (9,10). Fusion with a sodium carbonate—sodium hydroxide mixture or sodium maybe required if the samples are covalent or iasoluble. 

The biochemical basis for the toxicity of mercury and mercury compounds results from its ability to form covalent bonds readily with sulfur. Prior to reaction with sulfur, however, the mercury must be metabolized to the divalent cation. When the sulfur is in the form of a sulfhydryl (— SH) group, divalent mercury replaces the hydrogen atom to form mercaptides, X—Hg— SR and Hg(SR)2, where X is an electronegative radical and R is protein (36). Sulfhydryl compounds are called mercaptans because of their ability to capture mercury. Even in low concentrations divalent mercury is capable of inactivating sulfhydryl enzymes and thus causes interference with cellular metaboHsm and function (31—34). Mercury also combines with other ligands of physiological importance such as phosphoryl, carboxyl, amide, and amine groups. It is unclear whether these latter interactions contribute to its toxicity (31,36). 

Properties of zinc salts of inorganic and organic salts are Hsted in Table 1 with other commercially important zinc chemicals. In the dithiocarbamates, 2-mercaptobenzothiazole, and formaldehyde sulfoxylate, zinc is covalendy bound to sulfur. In compounds such as the oxide, borate, and sihcate, the covalent bonds with oxygen are very stable. Zinc—carbon bonds occur in diorganozinc compounds, eg, diethjizinc [557-20-0]. Such compounds were much used in organic synthesis prior to the development of the more convenient Grignard route (see Grignard reactions). 

Three different covalent cure systems are commonly used sulfur-based or sulfur donor, peroxide, and maleimide. These systems rely on a cross-linking agent and one or more accelerators to develop high cross-link density. 

The action of sulfur nucleophiles like sodium bisulfite and thiophenols causes even pteridines that are unreactive towards water or alcohols to undergo covalent addition reactions. Thus, pteridin-7-one smoothly adds the named S-nucleophiles in a 1 1 ratio to C-6 (65JCS6930). Similarly, pteridin-4-one (73) yields adducts (74) in a 2 1 ratio at C-6 and C-7 exclusively (equation 14), as do 4-aminopteridine and lumazine with sodium bisulfite. Xanthopterin forms a 7,8-adduct and 7,8-dihydropterin can easily be converted to sodium 5,6,7,8-tetrahydropterin-6-sulfonate (66JCS(C)285), which leads to pterin-6-sulfonic acid on oxidation (59HCA1854). 

The strain energies of these five-membered heterocycles are relatively small with values of 23.5, 24.8 and S.SkJmoF estimated for tetrahydrofuran, pyrrolidine and tetrahy-drothiophene respectively (74PMH(6)199). The closeness of the values for the two former compounds reflects the almost identical covalent radii of oxygen (0.66 A) and nitrogen (0.70 A) atoms. The sulfur atom with a much larger covalent radius of 1.04 A causes a... 

Mechanism Several possible pathways for the reaction have been proposed by Hasek et alP and by Martin and Kagan, one of which is presented here. It takes cognizance of the fact that a significant concentration of hydrogen fluoride is essential for the reaction. Since definite interaction or compound formation between covalent fluorides and sulfur tetrafluoride is known to... 

Reactions of ionic or covalent azides with chalcogen halides or, in the case of sulfur, with the elemental chalcogen provide an alternative route to certain chalcogen-nitrogen compounds. Eor example, the reaction of sodium azide with cyclo-Sa in hexamethylphosphoric triamide is a more convenient synthesis of S7NH than the S2CI2 reaction (Section 6.2.1). Moreover, the azide route can be used for the preparation of 50% N-enriched S7NH. 

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